M. Witanowski

Nitrogen NMR Spectroscopy

Publisher Summary This chapter focuses on Nitrogen nuclear magnetic resonance (NMR) spectroscopy. Significant developments are due to improvements in experimental techniques and instrumentation as well as due to an increased awareness of the utility of both Nitrogen-14 and Nitrogen-15 nuclei in NMR spectroscopy. In NMR spectroscopy, the main interest is in relative screening constants––that is, chemical shifts––both for different nuclei in the same molecule and for nuclei in different polyatomic molecules rather than in absolute screening constants. A theoretical approach that gives gauge-dependent results leads to difficulties when relative screenings for various nuclei are to be compared. A Nitrogen-14 NMR study of order fluctuations in the isotropic phase of liquid crystals has been reported. The experimental data for the isotropic phases of p-azoxyanisole and of diethylazoxy benzoate are accounted for in terms of short-range order fluctuations of the nematic and of the smectic types, respectively.

Nitrogen-14 nuclear magnetic resonance of azoles and their benzo-derivatives

Abstract The 14 N NMR spectra of some azoles, diazoles, triazoles, tetrazoles, oxazoles, thiazoles, and some bicyclic benzo-derivatives show a linear relationship between the chemical shifts and the SCF-PPP-MO π-charge densities. The resonances of the N-methyl nitrogen atoms are very characteristic of the type of ring system and may be used for distinguishing between tautomeric forms. The effects of tautomeric equilibria on the appearance of the 14 N spectra are considered. The average excitation energy approximation of the theory of chemical shifts is found to be reliable for the 14 N shifts of pyrrole-type nitrogen atoms in azole ring systems, even if the effect of polarisation in the σ-bonds is ignored.

External neat nitromethane scale for nitrogen chemical shifts

Abstract Precise NMR measurements of 14N screening constants relative to those of neat nitromethane as external standard in concentric spherical sample containers show that external and internal referencing systems which have so far been used are not suitable for nitrogen shift calibration with an accuracy of the order of 0.1 ppm. Accurate data are reported for recalculating nitrogen shifts referred to various external and internal standards. Neat nitromethane is recommended as a primary reference substance. The role of solvent and concentration effects on nitrogen chemical shifts is indicated for a few selected molecules.

Nitrogen-14 nuclear magnetic resonance of some six-membered aromatic heterocycles

Abstract The 14N NMR chemical shifts of some aromatic heterocycles including pyridine, 1,2-diazine. 1,3-diazine, 1,4-diazine, and 1,3,5-triazine systems in mono-, bi-, and tricyclic structures (azabenzene, azanaphthalene, and azaphenanthrene type) are shown to depend almost linearly on the π-charge density at the N atom obtained from P-P-P calculations. This dependence is considered in terms of various approximations within the general theory of chemical shifts. A similar correlation is observed for the pyridine ring containing various electron-donating mono-substituents. The 14N NMR shifts show that the three isomeric aminopyridines exist as such in acetone, whereas two of the corresponding hydroxy derivatives have essentially the pyridine-type structure. Careful re-examination of the δN scale of 14N chemical shifts shows that the zero chemical shift difference between the two primary internal standards, nitromethane and the nitrate ion, is correct as previously indicated rather than the difference of 3 ppm reported by other authors.

Nitrogen Chemical Shifts in Organic Compounds

The range of nitrogen chemical shifts in organic compounds is about 800 p.p.m. and may be even larger for paramagnetic complexes. However, the usefulness of the nuclear magnetic resonance spectra of any nucleus for structural investigations depends very much not only on the magnitude of the chemical shifts but also on the possibility of finding reasonable correlations of the shifts with molecular structure. Another important factor is the possible differentiation of closely related structures, such as those of isomeric compounds or molecules occurring in various chemical equilibria. Fortunately, a great deal has already been done by means of systematic investigation of organic compounds by nitrogen nuclear magnetic resonance, and rather simple empirical relationships between nitrogen chemical shift and molecular environment have been established for many important classes of such compounds. Theoretical explanations of observed chemical shifts usually provide a tool for probing deeply into the electronic structure of organic molecules.

14N NMR studies on some azolopyridines using a differential saturation method

Abstract The chemical shifts and signal half-height widths of the nitrogen nuclei in some azolopyridines are reported. By means of simple molecular orbital arguments the nitrogen nuclei at the junctions of the fused five- and six-membered conjugated rings (indolizine-type nitrogen atoms) are shown to reflect the electronic charge distribution within the azolopyridines studied. As indicated by the nitrogen NMR spectra the electronic structures of the five-membered rings, of the azolopyridines, are very similar to those of the parent azoles. A complete analysis of the complex 14 N NMR spectra, including severe signal overlap regions, has been performed by means of differential saturation effects for signals with different widths.

Nitrogen NMR studies on some fused ring N-heterocycles

Abstract15N and14N NMR data are reported for a number of fused ring N-heterocycles.14N line widths,15N-1H couplings and nitrogen shielding additivity rales are used for15N signal assignments. The results obtained lead to some progress in the generalization of the effects of nitrogen-nitrogen interactions on nitrogen shieldings.

13C Chemical shifts of some azaindolizines versus electron charge distribution

Abstract Azaindolizines, which contain all possible combinations of nitrogen atoms within the five-membered ring moiety, are used as models for the investigation of a relationship between electron charge distribution and 13C shifts. A linear correlation is observed between the shifts and total rather than π-charge densities as calculated by the INDO-MO method. The average excitation energy (AEE) approximation in the theory of nuclear screening is shown to hold separately for the CH moieties and the carbon atoms at the ring junction in indolizines. An empirical correlation with charge densities is obtained from the AEE method, as a result of the compensation of effects within the local paramagnetic term and the prevailing contribution to the latter of the effective nuclear charge.13C shifts afford a reasonable measure of the total net charges at the carbon atoms of indolizines. The INDO calculations indicate that the π-charges follow the pattern suggested by simple resonance structures but the overall charge density depends heavily on σ-core polarization effects.

A NITROGEN-14 NMR STUDY OF SOME OXIMES AND THEIR ISOMERIC STRUCTURES

Abstract The isomeric structures of oximes, nitrones and nitroso compounds may be readily differentiated on the basis of their nitrogen chemical shifts. The tautomeric equilibria present in oximenitroso systems are easily observed by means of 14N NMR spectroscopy. An explanation of the range of chemical shifts found within the oxime and nitrone groups of compounds is presented, based upon the average excitation energy approximation to the paramagnetic term in the nuclear screening tensor. The increase in screening of the nitrogen found upon protonation of an oxime is analogous to that observed for pyridine-type structures and their N-oxides.

14N nuclear magnetic resonance of some monosubstituted pyridines

The 14N chemical shifts of some monosubstituted pyridines are reported. They show a correlation with the electronic nature of the substituent which is not found with earlier 14N chemical shift data. The shifts may be satisfactorily explained in terms of the average excitation energy approximation if both π- and σ-bond polarization is considered by means of SCF-MO calculations and 14N nuclear quadrupole resonance data. The 14N N.M.R. line widths are discussed from the point of view of molecular reorientation processes in the liquid state.